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I am playing around to see whether and how I can upgrade the kernel of the LMDE2 64 from a 3.16 to a 4.x
As simple adding PPAs is not supported ( for UKUU ) I tried addingdeb http://ftp.debian.org/debian stable main contrib non-free
This gives me an enormous amount of packages to update and a similarly long list of packages stating The following packages have been kept back: ...
This worries me a little bit and I am not sure if it is a good idea to go for it. On the other hand I believe that this offers---in principle---a 4.x kernel.

No, that's definitely not a good idea. LMDE 2 is based on Debian 8 / Jessie which is currently old-stable. Adding stable repositories won't work well. LMDE 3 is in the works and that will use Debian 9 (currently stable) as a base.

If you want a newer version of something than what is available from the repositories you can often find it in the Jessie backports repository. Please add that one to your system instead. Follow the instructions for Jessie here: https://backports.debian.org/Instructions/

After adding backports to your system you can install linux-image-amd64 and linux-headers-amd64 (for 64 bit) or linux-image-686 and linux-headers-686 (for 32 bit) to your system with the apt-get command given on above linked page. That gives you kernel 4.9. There may be other packages you should install at the same time but I think you need at least these two from Jessie backports.

Maybe.
Since* it's possible to upgrade from Linux Mint 17.3 to 18 I think the team will make it possible to upgrade from LMDE 2 to 3 after a couple of weeks after the release of LMDE 3
* before it was not possible to do a decent upgrade.
Just my thoughts